1. Field of the Invention
The present invention relates to an ink jet recording ink, an ink jet recording method, an ink cartridge, and an ink jet-recording apparatus.
2. Related Background Art
The use of a dye as a recording agent (colorant in a recording solution) has conventionally involved a problem in that a so-called bronzing occurs. In the bronzing, the crystal of the dye is separated out on paper as a recorded image dries, and the recorded image reflects light to glow in a yellow or brown color, thereby emitting metallic luster. The occurrence of the bronzing involves the reflection of light, thereby leading to not only a reduction in density of the recorded image but also the considerable deviation of the color tone from that required for ink.
The addition of an alkanolamine such as N-hydroxyethylmorpholine, monoethanolamine, diethanolamine, or triethanolamine has been known to prevent the bronzing. However, the addition of even a small amount of any one of those alkanolamines to ink provides the ink with a high pH of 9 or more. An ink having such high pH is problematic in terms of ejection stability owing to, for example, the corrosion of a nozzle into which the ink is to be in contact. Furthermore, the ink involves a problem in terms of safety when a person erroneously touches the ink at the time of handling. An ink having a pH in a neutral region (a pH around 7) has been desired because the ink gives little damage when it erroneously enters an eye, and because it has a high degree of freedom of selection of a member to be in contact with the ink. The addition of any one of those alkanolamines to ink also involves a problem in that water resistance reduces even if the bronzing is suppressed.
The addition of paratoluenesulfonic amide ethylene oxide to ink had a certain effect on the bronzing, but reduced a contact-angle between the ink and a recording medium. As a result, feathering occurred, an optical density (OD) reduced, and hence printing quality remarkably deteriorated. The addition of a basic amino acid to ink has also been investigated. However, in this case, the addition amount must be at least 1 mass % to 2 mass %, and the ink may have a pH of 8 or more depending on the solvent composition of the ink although the pH does not become extraordinarily high. Accordingly, the ink cannot be said to be an ink having a pH in a neutral region. In addition, the corrosion or the like of a nozzle with which the ink is to be in contact has been concerned (see, for example, JP-A-No. H07-228810).
In recent years, a dye particularly excellent in weatherability has been used. However, in the case of a copper phthalocyanine dye in particular, a trade-off relation is established between the weatherability and the occurrence of the bronzing.
An ink used for an ink jet recording apparatus such as an ink jet printer is prepared by selecting an optimum material from a large number of materials. When attention is paid to, for example, a dye alone, the dye may be a direct dye, an acid dye, or the like each of which has different characteristics. For example, when printing is performed with an ink using a direct dye, a formed image is characterized in that it is relatively excellent in weatherability. When printing is performed with an ink using an acid dye, a printed product having high brightness and high chroma is obtained.
An ink jet recording ink is requested to have properties including the following properties. For example, (1) the ink does not cause clogging at the tip of a nozzle, (2) the ink can secure stable recording even at the time of continuous recording or even when recording is restarted after the ink has been left standing over a long period of time (the ink has start-up ejection stability or stick recoverability), and (3) the physical properties of the ink do not change even when the ink is stored for a long period of time.
The addition of urea, a derivative of urea, thiourea, a derivative of thiourea, or the like as a humectant to ink has been conventionally performed to prevent clogging of the tip of a nozzle due to the evaporation of water. The addition of urea or a derivative thereof as a solubilizing agent for a dye to ink has also been performed for increasing the solubility of the dye into water or various solvents with a view to improving start-up ejection stability (see, for example, JP-A-No. H02-173168).
However, some of the dyes may generate an aggregate or decompose owing to an ammonium ion generated by the decomposition of urea. Accordingly, the storage of an ink added with urea for a long period of time has involved a problem in that a precipitate is generated as a result of aggregation of the dye, and clogging occurs at an ink ejection port of an ink jet recording apparatus, in an ink supply nozzle thereof, and at any other location. Furthermore, an ink added with urea may cause a metal portion to be corroded by ammonia generated by the decomposition of urea regardless of the kind of a colorant, and the odor of the generated ammonia may provide a user with a sense of displeasure.
By the way, according to the conventional ink jet method, an ink having a viscosity as low as 1 to 3 mPa·s is ejected from nozzles or openings having small diameter of about 30 to 50 μm. In other words, the nozzles or openings can be replenished (refilled) with the ink at high speed to cope with a reduction in ink amount in the nozzles or openings due to the ejection of the ink because the ink has a sufficiently low viscosity. In this case, the meniscuses (the interface between the ink and the air) of the tip of the nozzles or the openings are recovered quickly, whereby ejection stability can be obtained even when high speed printing with a repetitive printing frequency in excess of 10 kHz is performed.
However, when a image is printed on a recording medium such as plain paper by means of an ink having a low viscosity, feathering occurs along the fiber of the paper because the ink penetrates into the recording medium quickly. The feathering itself provides a sense of roughness to reduce image quality. In addition, when adjacent dots are coupled owing to feathering, only such portion has reduced resolution, thereby causing a problem in that image quality extremely reduces. To solve those problems, the penetration rate of an ink into a recording medium is suppressed by, for example, controlling the surface tension of the ink. However, in the case where a color image is formed by means of an ink with the suppressed penetration rate, when inks having different colors come into contact with each other as a result of allowing them to impinge on a recording medium, color mixing feathering (bleeding) occurs to thereby significantly reduce image quality.
With regard to the above-described problems occurring when an ink having a low viscosity is used, an ink having a high viscosity is used to reduce the penetration rate of the ink into a recording medium, whereby feathering can be prevented. In this case, mixing rate of different colors is also suppressed, so color mixing can be prevented. Therefore, the use of an ink having a high viscosity is very useful in solving the above problems.
As a recording apparatus using an ink jet method has become widespread, the applications of the apparatus have been rapidly expanding. With the advent of the expansion, kinds of recording media have also become miscellaneous. The recording apparatus is further expected to be applicable to liquid nonabsorbing media such as metal, plastic, and glass in addition to paper. Therefore, the realization of an ink jet recording apparatus capable of using an ink having a high viscosity has been demanded.
However, in the conventional ink jet method, a flow path resistance increases as the viscosity of ink increases, with the result that the supply rate of an ink to nozzles or openings remarkably reduces. In this case, energy necessary for ejecting the ink also remarkably increases in accordance with the increase in the flow path resistance. For example, when an ink having a high viscosity is used in an ink jet apparatus that generally uses a commercially available aqueous ink (having a viscosity of 1 to 3 mPa·s), a nozzle or opening cannot be supplied with the ink without delay, and meniscuses cannot be recovered without delay when the viscosity of the ink exceeds about 10 mPa·s.
When one attempts to ejection an ink in such state, the ink is ejected from an unstable meniscus position. As a result, fluctuations in the ejection amount and ejection direction of the ink enlarge to reduce image quality. Furthermore, nozzles or openings cannot be supplied with the ink without delay when the viscosity of the ink exceeds 20 mPa·s, with the result that omission of ejection occurs. Furthermore, when the viscosity of the ink exceeds 100 mPa·s, even if the nozzles or openings can be supplied with the ink without delay, conventional pressure generating means does not generate sufficient energy for ejecting the ink, with the result that the ink is ejected unstably or is not ejected at all.
In view of the above, a printer intended for performing printing at high speed, which has a large nozzle diameter of about 50 μm to 70 μm to reduce a flow path resistance and uses an ink having a viscosity as high as 10 to 100 mPa·s, has been proposed (see, for example, JP-A-No. H09-169111). In this method, the supply rate of the ink reduces unless the nozzle diameter is increased by an amount corresponding to an increase in viscosity of the ink, so a repetitive printing frequency reduces. However, when the nozzle diameter is increased, the diameter of an ink droplet to be ejected relatively increases, thereby leading to reductions in resolution and image quality.